This invention generally concerns ceramic nozzle assemblies, and is specifically concerned with a composite ceramic nozzle for conducting a flow of liquid steel in an electromagnetic valve.
Metering nozzles for controlling the flow of molten metals in metal casting processes are well known in the prior art. Such metering nozzles typically comprise a nozzle formed from a ceramic material that has an open end of a fixed size. The rate of flow through the nozzle is a function of the size of the opening and the height of the molten metal above it. When such nozzles are used in a steel casting process, the flow of metal through the nozzle is controlled by means of a copper chill plug. Specifically, a copper chill plug is engaged against the open end of the nozzle when the operator of the nozzle desires to stop the flow of metal therethrough. The copper chill plug locally "freezes" the molten metal within the nozzle opening, and causes a solid plug of metal to form within the nozzle which effectively supports the column of molten metal above it. To restart the flow, the operator lances the open end of the metering nozzle in order to melt the solid plug that was created by the application of the copper chill plug. Unfortunately, the use of such a lance can erode the ceramic material forming the nozzle thereby damaging it.
One of the most recent advances in molten metal flow control is the use of an electromagnetic coil formed around a ceramic nozzle. Such are known in the art as electromagnetic valves, and are capable of restarting a flow of molten metal by inductive heating without the use of a damaging lance. In addition to damage free restarting characteristics, such electromagnetic valves are well suited to modulate the flow of molten metal in both an open pour, or in higher quality casting practices which employ the use of inert gases over the molten metals during the casting process to protect the metals from atmospheric contaminants.
Unfortunately, the applicants have observed that the thermal properties of the ceramic materials typically used to construct the nozzles in such electromagnetic valves have significantly limited the usefulness of such valves. The preferred material for forming the metering nozzles in such valves is zirconia due to the extremely high erosion-resistant properties of this material. However, the advantageous erosion-resistive properties of zirconia is substantially negated by this material's tendency to crack when exposed to the thermal stresses associated with the stopping and starting of a flow of liquid metal through the nozzle. The applicants have observed that the unfortunate tendency for zirconia to crack under such thermal stress conditions is caused by the fact that zirconia's thermal coefficient of expansion sharply changes (i.e., decreases) at a temperature which is within the thermal gradient that the walls of the metering nozzle experience when molten metal initially flows over the inner walls of the nozzle. This abrupt change in the thermal coefficient of expansion creates tensile hoop stresses in the annular walls of the nozzle which are of a magnitude sufficient to create cracks within the walls of the nozzle which may propagate to the surface of the nozzle and thereby jeopardize the structural integrity of the nozzle walls.
While it may be possible to solve the cracking problem by increasing the thickness of the nozzle walls to an extent to where the tensile strength of the outermost portion of the walls was sufficient to counteract the tensile hoop stresses created by the thermal gradient, such wall-thickening would logarithmically reduce the efficacy of the induction coil in interacting with and melting the metal plug in the nozzle during a restart operation since field strength would decrease as a function of the square of the radial distance between the induction coil and the metal plug. Thus, considerably more electrical power would have to be used in the electromagnetic valve in order to melt the metal plug to restart a molten flow, which in turn would require thicker and more expensive coil windings. But even if the problem of lowered electromagnetic efficiency were not present, the mere thickening of the zirconia walls of the nozzle could well fail to solve the problem, since the temperature gradient imposed across even thickened walls could still cause at least microcracking to occur at the interface in the wall where the thermal coefficient of expansion of the material changed. If there were any faults in the grain structure of the zirconia forming the nozzle, these micro-cracks might form fissures which would propagate entirely through the wall thickness of the nozzle, thereby jeopardizing the integrity of the nozzle. While the nozzle could be formed from a ceramic material having better thermal shock properties (such as boron nitride), none of these ceramic materials has anywhere near the erosion-resistant properties of zirconia when exposed to an environment of molten steel. The much faster erosion of the internal surface of such a nozzle would make it difficult if not impossible to accurately modulate the flow through the valve during its operation, and would also necessitate frequent (and expensive) nozzle replacements.
Clearly, there is a need for a nozzle assembly for use within a metal casting electromagnetic valve which maintains all of the anti-erosion properties of zirconia, but which is not prone to cracking when exposed to the thermal shock which occurs when molten metal is initially conducted through the interior of the nozzle.